12.Snow has dropped in this area since last December -Snow started
16.We have waited for the bus for an hour
-We started
17.His sister has sung karaoke since he went to school this morning
-His sister began
18.We haven't communicated with each other by radio for a long time
-We stopped
19. The dog has barked at that strange man since he arrived here
-The dog started
20.You haven't helped me since you graduated from college
-You last